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Correlation Between Day 0 Post-Implant Dosimetry Parameters and Differences in Pre- and Post-Implant Prostate Volume-How Much Can Post-Plan Quality Be Improved Through Better Assessment of Volume for Permanent Interstitial I-125 Prostate Implants?

Purpose: To evaluate the relationship between the difference in pre- and post-implant prostate volume and post-implant dosimetric parameters for pre-planned permanent interstitial prostate implants.

Methods: Between January 2007 and December 2011, 964 patients underwent TRUS guided interstitial prostate I-125 brachytherapy either as monotherapy (MPD 144Gy) or in combination with external irradiation (MPD 108Gy). All procedures and contouring for pre- and post-planning was performed by a single physician. Post-implant dosimetric analysis was performed on CTs taken on the day of implant. Differences in prostate volume as determined by TRUS and CT were evaluated by computing a parameter, Δvol, defined as the TRUS prostate volume (VTrus) minus the CT prostate volume (VCT) divided by VTrus. Implants were grouped into the following categories based on their post-implant dosimetry: 1.Hot (D90>120% and V150>70%); 2. Cold (D90<80%) and; 3.) Standard. Differences in Δvol across groups were evaluated. D90, V100 and V150 were plotted against their corresponding Δvol values.

Results: Overall, the average and standard deviation of Δvol, D90, V100 and V150 was 0.0%+/-17.6, 101%+/-12.9, 89.3%+/- 7.0, and 47%+/-12.8, respectively. For the standard, hot, and cold groups, the average and standard deviation of Δvol was 0.3%+/-15.5, -18.0%+/-9.3 and 26.6%+/-24.0, respectively. Plots of the dosimetric parameters versus Δvol show wide dispersion, for example R^2 = 0.422 for the D90 plot, but they strongly suggest a downward slope, highlighting the fact that cases with a large difference between VTrus and VCT are more likely to have nonstandard dosimetry than cases with a small difference in these volumes. It is noted that there are cases with Δvol close to 0 that have nonstandard post-plan dosimetry.

Conclusion: Our work demonstrates that limiting variation in Δvol may reduce but not eliminate the occurrence of nonstandard post-plans.